1. Field of the Invention
The present invention relates to a method for adjusting the length of a nanotube and more particularly to a nanotube length control method in which the length of a nanotube is controlled by disposing the nanotube in a state in which the tip end of the nanotube protrudes and cutting down the tip end of the nanotube by means of an electrical discharge.
2. Prior Art
Carbon nanotubes are typical examples of nanotubes.
Conventionally, carbon nanotubes have been manufactured by the arc discharge method and the vapor-phase growth method. However, carbon nanotubes manufactured by these methods have a wide distribution of diameters and lengths.
Meanwhile, the inventors of the present application have noted that carbon nanotubes have a high aspect ratio (length/diameter), superior flexibility and a high strength. As a result, the inventors envisioned a broad range of applications of such nanotubes as manipulating means in the nano-scale region. Such applications include, for instance, the use of carbon nanotubes as AFM (atomic force microscope) probe needles and the use of carbon nanotubes as input-output probe heads of magnetic devices. Consequently, the inventors proposed numerous devices that use such carbon nanotubes.
Nanotubes thus can be used as nano-scale manipulating means in a wide range of applications. In order to develop this theme in more concrete terms, a description will be made below for a nanotube used as an AFM probe needle as an example.
Generally, a cantilever made of a semiconductor which has a sharp protruding portion at the tip end is used as an AFM probe needle. The inventors of the present application developed a technique in which the base end portion of a carbon nanotube is firmly fastened to the protruding portion of such a cantilever by means of a coating film and/or thermal fusion. In recent years, techniques that allow a carbon nanotube to grow on a holder such as a cantilever by a CVD process (chemical vapor deposition process) have also been developed.
Such a nanotube probe is characterized in that it has superior physical properties of carbon nanotubes. In other words, the nanotube probe has high durability and shows almost no breakage even if the tip end portion of the carbon nanotube is used as a probe needle that scans the surface of a substance. Furthermore, since carbon nanotubes have a high aspect ratio with a diameter ranging from several nanometers to several tens of nanometers and a length ranging from a nanometer to micron range, an AFM probe needle with an extremely high resolution can be realized.
However, such nanotube probes have some weak points. In the case of an arc discharge method or vapor-phase growth method, as described above, the lengths and diameters of the carbon nanotubes that are manufactured have a wide distribution, and no manufacturing method that produces carbon nanotubes with a uniform diameter and length has yet been realized.
Thus, when a carbon nanotube is fastened to or grown on the protruding portion of a cantilever, the length of the tip end portion of the carbon nanotube that protrudes forward from the protruding portion of the cantilever tend unavoidably to be non-uniform. In cases where the length of the tip end portion of the carbon nanotube is not uniform, the performance of the nanotube probe varies, resulting in that a stable, uniform probe needle performance cannot be obtained.
For example, in cases where the length of the tip end portion of the nanotube is long, this tip end portion tends to undergo self-excited vibration. When the tip end portion vibrates, the tip end cannot detect information regarding various points on the sample surface with good precision when the sample surface is scanned. As a result, the resolution drops abruptly. In order to suppress self-excited vibration of the nanotube, it is necessary to shorten the length of the tip end portion of the nanotube to a specified length or less.
When a case in which a carbon nanotube is fastened to the protruding portion of a cantilever is considered, this fastening work is performed while viewing an enlarged image of the object under an electron microscope. If there is variation in the length of the carbon nanotube, it might be thought that it is sufficient to fasten the nanotube to the protruding portion by means of a coating film or thermal fusion after adjusting the length of the base end portion so that the length of the tip end portion is fixed.
However, fastening the carbon nanotube in place is performed under an electron microscope. Thus, the visual field is narrow, and it is often difficult to observe the base end portion of the carbon nanotube on the protruding portion of the cantilever. As a result, it has been almost impossible in the conventional nanotube probe manufacturing methods to eliminate variations in the length of the tip end portion of the carbon nanotubes. When the carbon nanotube is grown in a vapor phase on the protruding portion of the cantilever, the length is regulated by controlling the growth of nanotubes; accordingly, there is additional difficulty in such a length control.
The above-described drawbacks are also encountered in nanotubes other than carbon nanotubes, e.g., BN type nanotubes (boron-nitrogen type nanotubes) and BCN type nanotubes (boron-carbon-nitrogen type nanotubes). Furthermore, the drawbacks described above are not limited to AFM nanotube probes in which a nanotube is fastened to the protruding portion of a cantilever. Such drawbacks are also widely encountered in cases where nanotubes are fastened to the holders of devices that use nanotubes.
Accordingly, the object of the present invention is to provide a nanotube length control method which makes it possible to adjust the length of the tip end portion of a nanotube protruding from a holder to a desired length by cutting down the tip end portion of the nanotube.
The present invention is a nanotube length control method characterized in that:
a discharge needle and a nanotube that has a tip end portion which is protruded are set so that a tip end of the discharge needle is disposed to face a tip end of the nanotube,
a voltage is applied across the nanotube and the discharge needle, thus causing a discharge to occur between the tip end of the nanotube and the tip end of the discharge needle, and
cutting down the tip end of the nanotube by the discharge, thus controlling a length of the nanotube.
In the above method, the voltage is a direct-current voltage or a pulse voltage.
Also, the discharge needle is fastened to a metal plate so that the tip end thereof is protruded, the base end portion of the nanotube is fastened to a holder, and the voltage is applied across this metal plate and the holder.
Furthermore, the discharge needle is made of a nanotube.